Photodiode array

ABSTRACT

A photodiode array 1 has a plurality of photodetector channels 10 which are formed on an n-type substrate 2 having an n-type semiconductor layer 12, with a light to be detected being incident to the plurality of photodetector channels 10. The photodiode array 1 comprises: a p−-type semiconductor layer 13 formed on the n-type semiconductor layer 12 of the substrate 2; resistors 4 each of which is provided to each of the photodetector channels 10 and is connected to a signal conductor 3 at one end thereof; and an n-type separating part 20 formed between the plurality of photodetector channels 10. The p−-type semiconductor layer 13 forms a pn junction at the interface between the substrate 2, and comprises a plurality of multiplication regions AM for avalanche multiplication of carriers produced by the incidence of the light to be detected so that each of the multiplication regions corresponds to each of the photodetector channels. The separating part 20 is formed so that each of the multiplication regions AM of the p−-type semiconductor layer 13 corresponds to each of the photodetector channels 10.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is application is a continuation application of U.S. patentapplication Ser. No. 15/293,784, filed Oct. 14, 2016, which is acontinuation of U.S. patent application Ser. No. 13/774,002, filed Feb.22, 2013, now U.S. Pat. No. 9,484,366, which is a continuation of U.S.patent application Ser. No. 13/116,525, filed May 26, 2011, now U.S.Pat. No. 8,610,231, which is a division of U.S. patent application Ser.No. 12/306,963, filed Feb. 20, 2009, now U.S. Pat. No. 8,008,741, whichis National Stage Entry of PCT/JP2007/063299, filed Jul. 3, 2007, whichclaims priority to Japanese Patent Application No. 2006-183598, filedJul. 3, 2006, the entire contents of each of which is incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to a photodiode array.

BACKGROUND ART

In the fields of chemistry and medical care for example, there has beena technology for photon counting which uses a scintillator coupled witha photodiode array based on avalanche (electron avalanche)multiplication phenomenon. Such a photodiode array has dividedphotodetector channels formed on a common substrate in order todiscriminate a plurality of photons which simultaneously enter thereto,and each of the photodetector channels is provided with multiplicationregions (for example, see Non-Patent Documents 1 and 2, and PatentDocument 1).

Each multiplication region which is very sensitive to faint light iscaused to be operated under an operation condition, called Geiger mode.That is, each multiplication region is applied with a reverse voltagethat exceeds a breakdown voltage, and then the phenomenon is used inwhich carriers generated by the incident photons are multiplied in anavalanche process. Each photodetector channel is connected with aresistor for deriving an output signal from the multiplication region,and the resistors are connected with each other in parallel. The photonsincident to each photodetector channel are detected based on the peakvalue of the output signal which was derived outside through eachresistor.

-   [Patent Document 1] Japanese Patent Application Laid-Open No.    11-46010-   [Non-Patent Document 1]    P. Buzhan, et al., “An Advanced Study of Silicon Photomultiplier”    [online], ICFA Instrumentation BULLETIN Fall 2001 Issue, [searched    on 4 Nov. 2004], <URL: http://www.slac.stanford.edu/pubs/icfa/>-   [Non-Patent Document 2]    P. Buzhan, et al., “Silicon Photomultiplier And Its Possible    applications”, Nuclear Instruments and Methods in Physics Research A    504 (2003) 48-52

DISCLOSURE OF THE INVENTION

When a photodiode array is used for photon counting, in order to obtainsatisfactory result, it is important to increase detection efficiency byincreasing the ratio of opening area to a light to be detected.

However, for example, in the reach-through photodiode arrays describedin Non-Patent Documents 1 and 2, the pn junction of each photodiode isattained using a semiconductor layer which is formed on the surface ofeach photodetector channel. As a result, the semiconductor layer of theeach semiconductor layer for attaining a pn junction is required to haveguard ring at its outer periphery edge portion to prevent edgebreakdown, which limits any increase of the ratio of opening area to alight to be detected. In such a photodiode array having a low ratio ofopening area, the detection sensitivity is hardly improved. In addition,the avalanche multiplication emits a light which is absorbed by adjacentphotodetector channels, resulting in another problem of crosstalk.

The present invention was made to solve the above problems, and oneobject of the present invention is to provide a photodiode array havinga high ratio of opening area to a light to be detected.

In order to achieve the above object, a photodiode array according tothe present invention is the one in which a plurality of photodetectorchannels are formed on a substrate having a semiconductor layer of afirst conductivity type, with a light to be detected being incident tothe plurality of photodetector channels, characterized in that itcomprises: the substrate; an epitaxial semiconductor layer of a secondconductivity type which is formed on the semiconductor layer of thefirst conductivity type of the substrate and forms a pn junction at theinterface with the semiconductor layer, and also has a plurality ofmultiplication regions for avalanche multiplication of carriers producedby the incidence of the light to be detected so that each of themultiplication regions corresponds to each of the photodetectorchannels; and a plurality of resistors each of which has two ends and isprovided for each of the photodetector channels so as to be electricallyconnected to the epitaxial semiconductor layer via one end thereof andbe connected to a signal conductor via the other end thereof.

In the above photodiode array, the pn junction is configured with thesemiconductor layer of the first conductivity type of the substrate andthe epitaxial semiconductor layer formed on the semiconductor layer. Themultiplication regions are formed in the epitaxial semiconductor layerby which a pn junction is attained, and the multiplication regionscorresponding to each of the photodetector channels are comprised in theepitaxial semiconductor layer. Therefore, the above photodiode arraydoes not have any end (edge) of a pn junction where edge breakdown iscaused when the photodiode array is caused to be operated in Geigermode, and does not have to be provided with a guard ring. This allowsthe above photodiode array to have a higher ratio of opening area.

A photodiode array according to present invention is the one in which aplurality of photodetector channels are formed on a substrate having asemiconductor layer of a first conductivity type, with a light to bedetected being incident to the plurality of photodetector channels,characterized in that it comprises: the substrate; an epitaxialsemiconductor layer of the first conductivity type which is formed onthe semiconductor layer of the first conductivity type of the substrateand has a plurality of multiplication regions for avalanchemultiplication of carriers produced by the incidence of the light to bedetected so that each of the multiplication regions corresponds to eachof the photodetector channels; a diffusion region of a secondconductivity type which is formed in the epitaxial semiconductor layerof the first conductivity type and forms a pn junction at the interfacewith the epitaxial semiconductor layer; and a plurality of resistorseach of which has two ends and is provided for each of the photodetectorchannels so as to be electrically connected to the diffusion region ofthe second conductivity type in the epitaxial semiconductor layer viaone end thereof and be connected to a signal conductor via the other endthereof.

In the above photodiode array, a pn junction is formed by the epitaxialsemiconductor layer of the first conductivity type on the substrate andthe epitaxial semiconductor layer of the second conductivity type formedin the former semiconductor layer. Also, the multiplication regions areformed in the epitaxial semiconductor layer by which the pn junction isattained, and the multiplication regions corresponding each of thephotodetector channels are comprised in the epitaxial semiconductorlayer. Therefore, the above photodiode array does not have any end(edge) of a pn junction where edge breakdown is caused when thephotodiode array is caused to be operated in Geiger mode, and does nothave to be provided with a guard ring. This allows the above photodiodearray to have a higher ratio of opening area.

In order to arrange each multiplication region of the epitaxialsemiconductor layer in correspondence to each photodetector channel, thephotodiode array preferably further comprises a separating part of thefirst conductivity type which is formed between the plurality ofphotodetector channels. That is, a photodiode array according to presentinvention in which a plurality of photodetector channels are formed on asubstrate having a semiconductor layer of a first conductivity type,with a light to be detected being incident to the plurality ofphotodetector channels, preferably comprises: the substrate; anepitaxial semiconductor layer of a second conductivity type which isformed on the semiconductor layer of the first conductivity type of thesubstrate and forms a pn junction at the interface with the substrate,and also has multiplication regions for avalanche multiplication ofcarriers produced by the incidence of the light to be detected; aplurality of resistors each of which has two ends and is provided foreach of the photodetector channels so as to be electrically connected tothe epitaxial semiconductor layer via one end thereof and be connectedto a signal conductor via the other end thereof; and a separating partof the first conductivity type which is formed between the plurality ofphotodetector channels so that a plurality of multiplication regions ofthe epitaxial semiconductor layer are formed individually correspondingto each of the photodetector channels.

In the case, the separating part formed between the channels embodiesthe correspondence between each of the multiplication regions and eachof the photodetector channels. As a result, the photodiode array doesnot have to be provided with a guard ring, which allows the photodiodearray to have a higher ratio of opening area. In addition, theseparating part between the photodetector channels enables to wellrestrain crosstalk.

The separating part preferably comprises a light shielding part formedof a material which absorbs or reflects a light of a wavelength bandwhich is detected by the photodetector channels. Alternatively, theseparating part preferably comprises a light shielding part formed of amaterial which has a lower refractive index than that of the epitaxialsemiconductor layer. In these cases, the light is absorbed or reflectedby the light shielding part, which enables to well restrain thegeneration of crosstalk. In addition, the light shielding part ispreferably formed of a material which absorbs or reflects the light of awavelength band which is detected by the photodetector channels,especially the light of an invisible to near infrared wavelength bandwhich is generated by avalanche multiplication, in order to prevent theinfluence of the light emission due to avalanche multiplication onto theadjacent photodetector channels. This allows the generation of crosstalkto be well restrained.

The signal conductor is preferably formed above the separating part. Inthe case, this prevents the signal conductor from passing across a lightdetecting surface, which further improves the ratio of opening area.

Preferably the signal conductor is formed of aluminum, and is formed ona silicon nitride film. In the case, even when a high voltage is appliedto the photodiode array, the penetration of aluminum into the underlyingfilm can be restrained. The term “penetration” as used herein meansdiffusion and invasion, and will be used for the same meaning in thefollowing description. Furthermore, in the case, preferably theresistors are formed of polysilicon for example and are formed on asilicon dioxide film, and also have a silicon nitride film and a signalconductor formed thereon.

According to the present invention, a photodiode array having a highratio of opening area to a light to be detected can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically showing an upper surface of a photodiodearray according to a first embodiment;

FIG. 2 is a view showing a part of the cross section taken along theline II-II of the photodiode array according to the first embodiment;

FIG. 3 is a view schematically illustrating the connection between eachphotodetector channel and signal conductor and resistors;

FIG. 4 is a cross sectional view showing a first modification of thephotodiode array according to the first embodiment;

FIG. 5 is a cross sectional view showing a second modification of thephotodiode array according to the first embodiment;

FIG. 6 is a cross sectional view showing a photodiode array according toa second embodiment;

FIG. 7 is a view schematically illustrating a cross sectional structureof a photodiode array according to a third embodiment;

FIG. 8 is a cross sectional view showing a photodiode array according toa fourth embodiment;

FIG. 9 is a cross sectional view showing a photodiode array according toa modification of the layer structure of the embodiment shown in FIG. 2;

FIG. 10 is a cross sectional view showing a photodiode array accordingto a modification of the layer structure of the embodiment shown in FIG.4;

FIG. 11 is a cross sectional view showing a photodiode array accordingto a modification of the layer structure of the embodiment shown in FIG.5;

FIG. 12 is a cross sectional view showing a photodiode array accordingto a modification of the layer structure of the embodiment shown in FIG.6;

FIG. 13 is a cross sectional view showing a photodiode array accordingto a modification of the layer structure of the embodiment shown in FIG.7;

FIG. 14 is a cross sectional view showing a photodiode array accordingto a modification of the layer structure of the embodiment shown in FIG.8; and

FIG. 15 is a cross sectional view showing the part around asemiconductor layer 12.

DESCRIPTION OF SYMBOLS

-   1, 30, 40, 50 photodiode array-   2 substrate-   3 signal conductor-   4 resistor-   5 electrode pad-   10 photodetector channel-   11 insulator-   12 n⁺-type semiconductor layer-   13 p⁻-type semiconductor layer-   14 p⁺-type semiconductor layer-   15 p-type semiconductor layer-   16 protective film-   20 separating part-   22 light shielding part-   S substrate member-   AM multiplication region-   R13 n-type semiconductor layer-   R15 n-type semiconductor layer

BEST MODES FOR CARRYING OUT THE INVENTION

Now, with reference to the accompanying drawings, preferred embodimentswill be explained below in detail. In the following explanation, thesame elements or the elements having the same function are denoted bythe same reference numeral, and the duplicated explanation of theelements will be avoided.

First Embodiment

With reference to FIG. 1 and FIG. 2, the configuration of a photodiodearray 1 according to a first embodiment will be explained below. FIG. 1is a view schematically showing an upper surface of the photodiode array1 according to a first embodiment. FIG. 2 is a view showing a part ofthe cross section taken along the line II-II of the photodiode array 1according to the first embodiment.

The photodiode array 1 is formed with a plurality of semiconductorlayers and an insulating layer laminated to a substrate 2. As shown inFIG. 1, the photodiode array 1 is a multichannel avalanche photodiodefor photon counting having a plurality of photodetector channels 10formed in a matrix (4×4 in the present embodiment) to which a light tobe detected is incident. The photodiode array 1 has a signal conductor3, resistors 4, and an electrode pad 5 on the upper surface thereof. Thesubstrate 2 is a square with a side of about 1 mm, for example. Each ofthe photodetector channels 10 is a square, for example.

The signal conductor 3 comprises a reading part 3 a for transmittingsignals outputted from each photodetector channel 10, a connecting part3 b for connecting between each of the resistors 4 and the reading part3 a, and a channel surrounding part 3 c which is arranged to surroundthe outer periphery of each photodetector channel 10. The reading part 3a is connected to each of the photodetector channels 10 which aredisposed in the two columns adjacent to the reading part 3 a, and isconnected to the electrode pad 5 at one end thereof. In the presentembodiment, because the photodiodes are arranged in a matrix of 4×4,there are two reading parts 3 a arranged on the photodiode array 1, andboth of the reading parts 3 a are connected to the electrode pad 5. Thesignal conductor 3 is formed of aluminum (Al), for example.

The resistor 4 is provided to each of the photodetector channels 10 viaone end 4 a of the resistor 4 and the channel surrounding part 3 c, andis connected to the reading part 3 a via the other end 4 b of theresistor 4 and the connecting part 3 b. The plurality of resistors 4(eight in the present embodiment) connected to a common reading part 3 aare connected to the reading part 3 a. The resistor 4 is formed ofpolysilicon (Poly-Si), for example.

Next, with reference to FIG. 2, the cross sectional configuration of thephotodiode array 1 will be explained below. As shown in FIG. 2, thephotodiode array 1 comprises the substrate 2 having a semiconductorlayer of n-type (a first conductivity type), a p⁻-type semiconductorlayer 13 of p-type (a second conductivity type) which is formed on thesubstrate 2, a p⁺-type semiconductor layer 14 of p-type which is formedon the p⁻-type semiconductor layer 13, a protective film 16, aseparating part 20 of n-type (the first conductivity type) which isformed on the p⁻-type semiconductor layer 13, and the above signalconductor 3 and the resistors 4 formed on the protective film 16. Thelight to be detected is incident from the upper surface side shown inFIG. 2.

The substrate 2 comprises a substrate member S, an insulator 11 formedon the substrate member S, and an n⁺-type semiconductor layer 12 formedon the insulator 11. The substrate member S is formed of Si (silicon),for example. The insulator 11 is formed of SiO₂ (silicon dioxide), forexample. The n⁺-type semiconductor layer 12 is formed of Si for example,and is a semiconductor layer of n-type having a high impurityconcentration.

The p⁻-type semiconductor layer 13 is an epitaxial semiconductor layerof p-type having a low impurity concentration. The p⁻-type semiconductorlayer 13 forms a pn junction at the interface between the substrate 2.The p⁻-type semiconductor layer 13 has a plurality of multiplicationregions AM for avalanche multiplication of carriers that are produced bythe incidence of the light to be detected, corresponding to each of thephotodetector channels 10. The p⁻-type semiconductor layer 13 has athickness of 3 μm to 5 μm, for example. The p⁻-type semiconductor layer13 is formed of Si, for example.

The p⁺-type semiconductor layer 14 is formed on the p⁻-typesemiconductor layer 13 in correspondence to the multiplication region AMof each photodetector channel 10. That is, the area near the interfacebetween the substrate 2 and the p⁻-type semiconductor layer 13 which ispositioned below the p⁺-type semiconductor layer 14 in the direction ofthe lamination of semiconductor layer (hereinafter, simply referred toas lamination direction) is the multiplication region AM. The p⁺-typesemiconductor layer 14 is formed of Si, for example.

The separating part 20 is formed between the plurality of photodetectorchannels 10 for separating the photodetector channels 10 from eachother. That is, the separating part 20 is formed so that onemultiplication region AM can be formed in the p⁻-type semiconductorlayer 13 corresponding to each of the photodetector channels 10. Theseparating part 20 is formed in a two-dimensional lattice structure onthe substrate 2 to completely surround each of the multiplicationregions AM. The separating part 20 is formed from the upper surfacethrough to the lower surface of the p⁻-type semiconductor layer 13. Theseparating part 20 is a semiconductor layer of n-type having a highimpurity concentration, with the impurity being composed of P forexample. When the separating parts 20 is formed by diffusion, theprocess takes a long time of period, which may cause the impurity in then⁺-type semiconductor layer 12 to be diffused into the epitaxialsemiconductor layer to build a ridge at the interface of a pn junction.In order to prevent the ridge, a trench etch process may be performed onthe central part of the area for the separating parts 20 before theimpurity is caused to be diffused and the separating part 20 is formed.The details will be explained later in other embodiments, but the trenchgroove may be filled with a material which absorbs or reflects the lightof a wavelength band which the photodetector channel absorbs so as toform a light shielding part, so that crosstalk which is produced by theinfluence of the light emission due to avalanche multiplication onto theadjacent photodetector channels can be prevented.

The p⁻-type semiconductor layer 13, the p⁺-type semiconductor layer 14,and the separating part 20 form a flat plane on the upper surface sideof the photodiode array 1, and the protective film 16 is formed on theseelements. The protective film 16 is formed with an insulating layerwhich is formed of SiO₂, for example.

The protective film 16 has the signal conductor 3 and the resistor 4formed thereon. The reading part 3 a of the signal conductor 3 and theresistor 4 are formed above the separating parts 20.

The signal conductor 3 functions as an anode, and as a cathode, notshown, but a transparent electrode layer (for example, a layer formed ofITO (Indium Tin Oxide)) may be provided all over the lower surface side(the side without the insulating layer 11) of the substrate 2.Alternatively, as a cathode, the electrode part may be formed to beexposed to the surface side.

Now, with reference to FIG. 3, the connection between each photodetectorchannel 10 and the signal conductor 3 and resistor 4. FIG. 3 is a viewschematically illustrating the connection between each photodetectorchannel 10 and the signal conductor 3 and resistor 4. As shown in FIG.3, the p⁺-type semiconductor layer 14 of each photodetector channel 10and the signal conductor 3 (the channel surrounding part 3 c) aredirectly connected to each other. This makes the signal conductor 3 (thechannel surrounding part 3 c) and the p⁻-type semiconductor layer 13 beelectrically connected to each other. Also, the p⁻-type semiconductorlayer 13 and one end 4 a of the resistor 4 are connected via the signalconductor 3 (the channel surrounding part 3 c), while the resistors 4are individually connected to the reading part 3 a via the connectingpart 3 b at the other ends 4 b thereof.

When used for photon counting, the photodiode array 1 as configuredabove is caused to be operated under an operating condition, calledGeiger mode. In Geiger mode operation, each photodetector channel 10 isapplied with a reverse voltage (for example, 50 V or more) that exceedsa breakdown voltage. In the state, when a light to be detected isincident to each photodetector channel 10 from the upper surface sidethereof, the light to be detected is absorbed in the photodetectorchannel 10 to generate carriers. The generated carriers are acceleratedand moved according to the electric field in the photodetector channel10 for multiplication at each multiplication region AM. The multipliedcarriers are drawn outside through the resistor 4 by the signalconductor 3, which are detected based on the peak value of the outputtedsignal. The channels that detected photons individually provide an equalamount of output, thereby the detection of the total output of theentire channels enables the counting of the photodetector channels 10out of the photodiode array 1 which provided the outputs. Therefore, inthe photodiode array 1, one light emission of the light to be detectedenables the photon counting.

In the photodiode array 1, a pn junction is configured with the n⁺-typesemiconductor layer 12 of the substrate 2 and the p⁻-type semiconductorlayer 13 which is an epitaxial semiconductor layer formed on the n⁺-typesemiconductor layer 12 of the substrate 2. Also, the multiplicationregions AM are formed in the p⁻-type semiconductor layer 13 by which thepn junction is attained, and the correspondence of each multiplicationregion AM to each photodetector channel 10 is realized by the separatingparts 20 which are formed between the photodetector channels 10. The pnjunction has surfaces which are configured with the interface betweenthe n⁺-type semiconductor layer 12 and the p⁻-type semiconductor layer13 and the interface between the separating parts 20 and the p⁻-typesemiconductor layer 13, so that the region having a high impurityconcentration becomes convex and the region having a high electric fieldis eliminated. As a result, the photodiode array 1 does not have any end(edge) of a pn junction where edge breakdown is caused when thephotodiode array is caused to be operated in Geiger mode. Therefore, thephotodiode array 1 does not have to be provided with a guard ring forthe pn junction of each photodetector channel 10. This allows thephotodiode array 1 to have an extremely high ratio of opening area.

In addition, the high ratio of opening area allows the photodiode array1 to have a high detection efficiency.

Moreover, the photodetector channels 10 separated from each other by theseparating part 20 well restrain crosstalk.

In the operation in Geiger mode, even when the voltage differencebetween the photodetector channel with incident photons and that withoutphotons is large, the separating layer 20 formed between thephotodetector channels 10 sufficiently separate the channels.

In the photodiode array 1, the reading part 3 a of the signal conductor3 is formed above the separating parts 20. This prevents the signalconductor 3 from passing across above the multiplication region AM, thatis, on the light detecting surface, which further improves the ratio ofopening area. This is also considered to be effective in restrainingdark current. In the photodiode array 1, the resistors 4 are also formedabove the separating parts 20, which further improves the ratio ofopening area.

The applicant of the present invention has found, based on thewavelength dependency of afterpulse, that a problem arises that a partof holes generated at the n-type semiconductor substrate enter themultiplication region later and cause afterpulse when an n-typesemiconductor substrate is used and a p-type epitaxial semiconductorlayer is formed on the n-type semiconductor substrate. With respect tothe problem, in the photodiode array 1, for example, an insulator 11formed of SiO₂ is interposed between the substrate member S and then⁺-type semiconductor layer 12, so that the substrate member S and then⁺-type semiconductor layer 12 can be completely separated from eachother, which prevents afterpulse.

Various modifications can be applied to the separating parts 20 of thepresent embodiment. FIG. 4 is a cross sectional view showing a firstmodification of the photodiode array 1 according to the presentembodiment. In a photodiode array according to a first modification, aplurality of (two in the present modification) separating parts 20 areformed between the photodetector channels 10.

FIG. 5 is a cross sectional view showing a second modification of thephotodiode array 1 according to the present embodiment. In a photodiodearray according to a second modification, the separating parts 20 areformed only near the upper surface (the surface to which a light to bedetected is incident), and do not pass from the upper surface sidethrough to the lower surface side of the p⁻-type semiconductor layer 13in the lamination direction.

Also in the above embodiment, the epitaxial semiconductor layer is theone of a second conductivity type, but the epitaxial semiconductor layermay be the one of the first conductivity type that has a diffusionregion of the second conductivity type formed therein, so that a pnjunction is configured with the epitaxial semiconductor layer of thefirst conductivity type and the diffusion region of the secondconductivity type.

Second Embodiment

With reference to FIG. 6, the configuration of a photodiode array 30according to a second embodiment will be explained below. FIG. 6 is aview schematically illustrating the cross sectional structure of thephotodiode array 30 according to the second embodiment. The photodiodearray 30 according to the third embodiment is similar to the photodiodearray 1 according to the first embodiment except that the separatingpart 20 has a light shielding part.

As shown in FIG. 6, the separating parts 20 comprises a light shieldingpart 22 formed of a material which absorbs a light of a wavelength band(from invisible to near infrared) which is to be detected by thephotodetector channel 10. The light shielding part 22 is formed to beembedded in the separating part 20 like a core extending from the uppersurface side to the lower surface side of the p⁻-type semiconductorlayer 13. The light shielding part 22 is formed of a metal such as blackphoto-resist which is obtained by mixing a black dye or a pigment suchas insulation-processed carbon black into photoresist, and tungsten.However, when the light shielding part 22 is formed of a non-insulativematerial (for example, a metal such as tungsten), the light shieldingpart 22 has to be coated with an insulator such as SiO₂. As described inthe above first embodiment, when the separating part 20 is formed bydiffusion, the process takes a long time of period, which may cause theimpurity in the n⁺-type semiconductor layer 12 to be diffused to theepitaxial semiconductor layer and to build a ridge at the interface of apn junction. In order to prevent the ridge, a trench etch process may beperformed on the central part of the area for the separating parts 20before the impurity is caused to be diffused and the separating part 20is formed. As shown in FIG. 6, after the diffusion of impurity, then⁺-type semiconductor layer 12 and the separating part 20 are connectedto each other. The remaining trench groove may be filled with a materialwhich absorbs the light having a wavelength band the photodetectorchannels absorb as described above (as will be explained later, thegroove may be filled with a material which reflects the light of awavelength band which the photodetector channels absorb) so as to form alight shielding part, so that crosstalk which is produced by theinfluence of the light emission due to avalanche multiplication onto theadjacent photodetector channels can be prevented.

As in the case with the photodiode 1, the photodiode array 30 does nothave any end (edge) of a pn junction where edge breakdown is caused whenthe photodiode array is caused to be operated in Geiger mode. Therefore,the photodiode array 30 also does not have to be provided with a guardring for the pn junction of each photodetector channel 10. This allowsthe photodiode array 30 to have a high ratio of opening area.

In addition, the high ratio of opening area allows the photodiode array30 to have a high detection efficiency.

Moreover, the photodetector channels 10 which are separated from eachother by the separating parts 20 well restrain crosstalk.

In the photodiode array 30 also, the reading part 3 a of the signalconductor 3 is formed above the separating parts 20, which furtherimproves the ratio of opening area. This is also considered to beeffective in restraining dark current.

Furthermore, in the photodiode array 30 also, the insulator 11 isinterposed between the substrate member S and the n⁺-type semiconductorlayer 12, which enables to restrain afterpulse.

Also, each separating parts 20 comprises the light shielding part 22formed of a material which absorbs light of a wavelength band which isto be detected by the photodetector channel 10. As a result, the lightto be detected is absorbed at the light shielding part, which wellrestrains the generation of crosstalk. Moreover, the light shieldingpart 22 is formed of a material which absorbs the light of a wavelengthband which is detected by the photodetector channel 10, especially thelight of an invisible to near infrared wavelength band which isgenerated by avalanche multiplication, in order to prevent the influenceof the light emission due to avalanche multiplication onto the adjacentphotodetector channels 10, which well restrains the generation ofcrosstalk.

In addition to the material which absorbs an invisible to near infraredlight, the light shielding part 22 may be formed of a material whichreflects an invisible to near infrared light. Even in the case, becausethe light to be detected is reflected by the light shielding part, thegeneration of crosstalk can be well restrained. Moreover, the lightshielding part 22 is formed of a material which reflects the lighthaving a wavelength band of light detected by the photodetector channel10, especially the light having an invisible to near infrared wavelengthband which is generated by avalanche multiplication, in order to preventthe influence of the light emission due to avalanche multiplication ontothe adjacent photodetector channels 10, which well restrains thegeneration of crosstalk.

In addition to the material which absorbs or reflects an invisible tonear infrared light, the light shielding part 22 may be formed of amaterial which absorbs or reflects the light of a wavelength band whichis to be detected by the photodetector channel 10. However, the lightshielding part 22 is preferably formed of a material which absorbs orreflects the light of a wavelength band of light which is detected bythe photodetector channel 10, especially the light of an invisible tonear infrared wavelength band which is generated by avalanchemultiplication, in order to prevent the influence of the light emissiondue to avalanche multiplication onto the adjacent photodetector channels10.

The light shielding part 22 may be formed of a material having a lowerrefractive index than that of the separating parts 20. Even in thesecases, because a light is reflected by the light shielding part, thegeneration of crosstalk can be well restrained.

Third Embodiment

With reference to FIG. 7, the configuration of a photodiode array 40according to a third embodiment will be explained below. FIG. 7 is aview schematically illustrating the cross sectional structure of thephotodiode array 40 according to a third embodiment. The photodiodearray 40 according to the third embodiment is similar to the photodiodearray 1 according to the first embodiment except that the signalconductor 3 is formed on a silicon nitride film.

As shown in FIG. 7, the photodiode array 40 comprises the substrate 2having a semiconductor layer of n-type (a first conductivity type), thep-type semiconductor layer 15 of p-type (a second conductivity type)which is formed on the substrate 2, the p⁺-type semiconductor layer 14of p-type which is formed on the p-type semiconductor layer 15,protective films 16 a and 16 b, the separating part 20 of n-type (thefirst conductivity type) formed on the p-type semiconductor layer 15,the signal conductor 3 formed of aluminum, and resistors 4 formed ofPoly-Si for example. A light to be detected is incident from the upperside of FIG. 7.

The substrate 2 comprises an n⁺-type substrate member S, and an n-typesemiconductor layer 12 formed on the substrate member S.

The p-type semiconductor layer 15 is an epitaxial semiconductor layer ofp-type having a lower impurity concentration than that of the p⁺-typesemiconductor layer 14. The p-type semiconductor layer 15 forms a pnjunction at the interface between the n-type semiconductor layer 12 ofthe substrate 2. The p-type semiconductor layer 15 has a plurality ofmultiplication regions AM for avalanche multiplication of carriersproduced by the incidence of the light to be detected, corresponding toeach of the photodetector channels 10. The p-type semiconductor layer 15is formed of Si, for example.

The p-type semiconductor layer 15, the p⁺-type semiconductor layer 14,and the separating part 20 form a flat surface on the upper surface sideof the photodiode array 40, and have the protective films 16 a and 16 bformed thereon. The protective film 16 a is formed of an insulator whichis composed of a silicon dioxide film (SiO₂ film), while protective film16 b is formed of an insulator which is composed of silicon nitride (SiNfilm or Si₃N₄ film).

As shown in FIG. 7, the separating parts 20 have the protective film 16a, the resistor 4, the protective film 16 b, and the signal conductor 3laminated thereon in the order. Specifically, the protective film 16 ais laminated to the separating parts 20. The resistor 4 is laminated tothe protective film 16 a. The protective film 16 b is laminated to theprotective film 16 a and the resistors 4, except a part of the resistors4. The signal conductor 3 is laminated for electrical connection on theprotective film 16 b and the resistor 4 having no laminated protectivefilm 16 b thereon. Specifically, the reading part 3 a of the signalconductor 3 is laminated between the resistors 4, and the signalconductor 3 is laminated for electrical connection to the resistors 4 asthe electrical connection to the connecting part 3 b or channelsurrounding part 3 c.

Moreover, as shown in FIG. 7, the p⁺-type semiconductor layer 14 has theprotective film 16 b laminated thereon except a part of the p⁺-typesemiconductor layer 14. On the part of the p₊-type semiconductor layer14 having no laminated protective film 16 b thereon and a part of theprotective film 16 b laminated to the p⁺-type semiconductor layer 14,the channel surrounding part 3 c of the signal conductor 3 is laminatedfor electrical connection.

As in the case with the photodiode 1, the photodiode array 40 does nothave any end (edge) of a pn junction where edge breakdown is caused whenthe photodiode array is caused to be operated in Geiger mode. Therefore,the photodiode array 40 also does not have to be provided with a guardring for the pn junction of each photodetector channel 10. This allowsthe photodiode array 40 to have a high ratio of opening area.

In addition, the high ratio of opening area allows the photodiode array40 to have a high detection efficiency.

Moreover, the photodetector channels 10 which are separated from eachother by the separating parts 20 well restrain crosstalk.

In the photodiode array 40 also, the reading part 3 a of the signalconductor 3 is formed above the separating parts 20, which furtherimproves the ratio of opening area. This is also considered to beeffective in restraining dark current.

When the signal conductor 3 composed of aluminum is formed on an oxidefilm for example, there arises a problem that the aluminum is caused topenetrate the underlying film upon an application of a high voltage.With respect to the problem, in the photodiode array 40, the signalconductor 3 is formed on the protective film 16 b which is composed of asilicon nitride film. As a result, even when a high voltage is appliedto the photodiode array 40, the penetration of aluminum into theunderlying film (protective film 16 b) can be restrained.

In addition, under the reading part 3 a of the signal conductor 3, theprotective film 16 b and the protective film 16 a or the resistors 4 arelaminated. This well restrains the penetration of the aluminum into theseparating parts 20 and the p-type semiconductor layer 15 upon anapplication of a high voltage.

In this way, in the photodiode array 40, even when a high voltage isapplied, the penetration of the aluminum into photodetector channel 10and the separating parts 20 is preferably restrained.

The resistors 4 formed of polysilicon (Poly-Si) for example are formedon the protective film 16 a, and also have the protective film 16 b andthe signal conductor 3 formed thereon.

Instead of the n-type semiconductor layer 12, a p-type semiconductorlayer may be used. In the case, a pn junction is configured between thep-type semiconductor layer and the n⁺-type substrate member S (substrate2), and the multiplication regions AM are formed in the p-typesemiconductor layer.

Fourth Embodiment

With reference to FIG. 8, the configuration of a photodiode array 50according to a fourth embodiment will be explained below. FIG. 8 is across sectional view showing the photodiode array 50 according to thefourth embodiment. The photodiode array 50 according to the fourthembodiment is similar to the photodiode array 1 according to the firstembodiment except that the photodiode array 50 does not comprise theseparating part 20.

As shown in FIG. 8, the p⁻-type semiconductor layer 13 has a pluralityof multiplication regions AM so that each of the multiplication regionsAM correspond to each of the photodetector channel 10. Between thephotodetector channels 10, the signal conductor 3 and the resistors 4are formed.

As in the case with the photodiode 1, the photodiode array 50 does nothave any end (edge) of a pn junction where edge breakdown is caused whenthe photodiode array is caused to be operated in Geiger mode. Therefore,the photodiode array 50 also does not have to be provided with a guardring for the pn junction of each photodetector channel 10. This allowsthe photodiode array 50 to have a high ratio of opening area. Moreover,the photodiode array 50 having no separating part is able to exhibit afurther higher ratio of opening area.

In addition, the high ratio of opening area allows the photodiode array50 to have a high detection efficiency.

In the photodiode array 50, the reading part 3 a of the signal conductor3 is formed between the photodetector channel 10, which further improvesthe ratio of opening area. This is also considered to be effective inrestraining dark current.

Moreover, in the photodiode array 50 also, the insulator 12 is providedbetween the substrate member S and the n⁺-type semiconductor layer 12,which enables to restrain afterpulse.

While, although the preferred embodiments and modifications of thepresent invention have been explained, the present invention is notlimited to the above embodiments and modifications, and various changescan be added thereto. For example, the number of the photodetectorchannels formed in the photodiode array is not limited to the one (4×4)in the above embodiment. Also, the number of the separating parts 20formed between the photodetector channel 10 is not limited to the oneshown in the above embodiments and modifications, and may be three ormore for example. The signal conductor 3 may not formed above theseparating parts 20. Also, the resistors 4 may not formed above theseparating parts 20. The layers and the like are not limited to thoseillustrated in the above embodiments and modifications.

Under the n-type semiconductor layer 12, a buffer layer formed of ann-type semiconductor may be used. Instead of the n-type semiconductorlayer 12, a p-type semiconductor layer may be used, under which thebuffer layer formed of an n-type semiconductor may be used. In the case,a pn junction is formed between the p-type semiconductor layer and then-type buffer layer, and a multiplication region AM is formed in thep-type semiconductor layer. Moreover, in the case without the insulator11 of the third embodiment, a p-type semiconductor layer may be usedinstead of the n-type semiconductor layer 12, under which a buffer layerformed of a p-type semiconductor may be used. In the case, a pn junctionis formed between the p-type buffer layer and the n⁺-type substratemember S (substrate 2), and a multiplication region AM is formed in thep-type buffer layer.

In the above described photodiode arrays, a plurality of avalanchephotodiodes (which are composed of pn junctions of the detector channels10) that operate in Geiger mode are two-dimensionally arranged, and theavalanche photodiodes comprise the highly crystalline epitaxialsemiconductor layer 13. The above described photodiode arrays comprisethe resistor 4 which is electrically connected to the avalanchephotodiode at one end (anode) thereof and is arranged on the lightincident surface of the avalanche photodiode; and the signal conductor 3which is connected to the other end of the resistor 4, and no guard ringis interposed between the adjacent avalanche photodiodes. In thestructure, there in no guard ring, which allows the opening areas ofdetector channels to be increased.

FIG. 9 is a cross sectional view showing a photodiode array according toa modification of the layer structure of the embodiment shown in FIG. 2;FIG. 10 is a cross sectional view showing a photodiode array accordingto a modification of the layer structure of the embodiment shown in FIG.4; FIG. 11 is a cross sectional view showing a photodiode arrayaccording to a modification of the layer structure of the embodimentshown in FIG. 5; FIG. 12 is a cross sectional view showing a photodiodearray according to a modification of the layer structure of theembodiment shown in FIG. 6; FIG. 13 is a cross sectional view showing aphotodiode array according to a modification of the layer structure ofthe embodiment shown in FIG. 7; and FIG. 14 is a cross sectional viewshowing a photodiode array according to a modification of the layerstructure of the embodiment shown in FIG. 8. These photodiode arrayshave the same basic flat plane configuration and connections as thoseshown in FIG. 1.

As described above, in the structure shown in FIGS. 9 to 14, instead ofthe p-type semiconductor layer 13 or p-type semiconductor layer 15 ofFIG. 2, FIG. 4, FIG. 5, FIG. 6, FIG. 7, and FIG. 8, an n-typesemiconductor layer R13 or R15 is used. In the case, a pn junction isformed at the interface between the n-type semiconductor layer R13 (orR15) having a low concentration and the p-type semiconductor layer 14,and a depletion layer extends from the pn junction toward the n-typesemiconductor layer R13 (or R15), and a multiplication region AMcorresponding to the depletion layer is formed from the pn junctioninterface toward the n-type semiconductor layer R13 (or R15). The otherstructures and operations are the same as those described above.

These photodiode array 1 is configured with the plurality ofphotodetector channels 10 which are formed on the n-type substrate 2having the n-type semiconductor layer 12 thereon, with a light to bedetected being incident to the photodetector channels 10. The photodiodearray in which a plurality of photodetector channels 10 are formed on asubstrate having a semiconductor layer 12 (S) of n⁺-type which is afirst conductivity type, with a light to be detected being incident tothe plurality of photodetector channels 10, comprises the substrate 2;the epitaxial semiconductor layer R13 (or R15) of n⁻-type, a firstconductivity type, which is formed on the first conductivity typesemiconductor layer 12 of the substrate 2 and has a plurality ofmultiplication regions AM for avalanche multiplication of carriersproduced by the incidence of the light to be detected so that each ofthe multiplication regions AM corresponds to each of the photodetectorchannels 10; a diffusion region 14 of P⁺-type which is a secondconductivity type, the region being formed in the epitaxialsemiconductor layer R13 (or R15) of the first conductivity type andforms a pn junction at the interface with the epitaxial semiconductorlayer R13 (or R15); and a plurality of resistors 4 each of which has twoends and is provided for each photodetector channel 10 so as to beelectrically connected to the diffusion region 14 of the secondconductivity type in the epitaxial semiconductor layer R13 (or R15) viaone end 4 a thereof, and be connected to the signal conductor 3 via theother end 4 b thereof.

Each of the resistors 4 is, as shown in FIG. 1, provided for eachphotodetector channel 10 via one end 4 a thereof and the channelsurrounding part 3 c, and is connected to the reading part 3 a via theother end 4 b thereof and the connecting part 3 b. The plurality ofresistors 4 connected to a common reading part 3 a are connected to thereading part 3 a.

In these photodiode arrays, a pn junction is formed with the epitaxialsemiconductor layer R13 (or R15) of the first conductivity type on thesubstrate and the epitaxial semiconductor layer 14 of the secondconductivity type formed in the epitaxial semiconductor layer R13 (orR15). A multiplication region AM is formed in the epitaxialsemiconductor layer R13 (or R15) by which a pn junction is attained, andthe multiplication region AM corresponding to each photodetector channelis positioned in the epitaxial semiconductor layer R13 (or R15).Therefore, the above photodiode array does not have any end (edge) of apn junction where edge breakdown is caused when the photodiode array iscaused to be operated in Geiger mode, and does not have to be providedwith a guard ring. This allows the above photodiode array to have a highratio of opening area.

FIG. 15 is a cross sectional view showing a buffer layer 12X which isprovided on the semiconductor layer 12. The buffer layer 12X is composedof an n-type semiconductor layer. On the n-type semiconductor layer 12,a buffer layer 12X composed of an n-type semiconductor may be used.Alternatively, on the n-type semiconductor layer 12, a buffer layer 12Xcomposed of a p-type semiconductor may be used. In the case, a pnjunction is formed between the n-type semiconductor layer 12 and thep-type buffer layer 12X, and a multiplication region AM is formed in thep-type buffer layer 12X. Moreover, in the case without the insulator 11of the third embodiment, a p-type semiconductor layer may be usedinstead of the n-type semiconductor layer 12, and a buffer layercomposed of p-type semiconductor may be used on the p-type semiconductorlayer. In the case, a pn junction is formed between the p-typesemiconductor layer and the n⁺-type substrate member S (substrate 2),and a multiplication region AM is formed in the p-type semiconductorlayer.

INDUSTRIAL APPLICABILITY

The present invention can be used as a photodiode array having a highratio of opening area to a light to be detected.

The invention claimed is:
 1. An avalanche photodiode array comprising aplurality of avalanche photodiodes operating under Geiger mode, eachavalanche photodiode of the avalanche photodiode array comprising: afirst semiconductor layer having a first conductive type; a secondsemiconductor layer disposed over at least a portion of the firstsemiconductor layer; a third semiconductor layer disposed over at leasta portion of the second semiconductor layer, wherein the thirdsemiconductor has a second conductive type different from the firstconductive type, wherein a pn junction is formed between one of i) thesecond semiconductor layer and the first semiconductor layer or ii) thesecond semiconductor layer and the third semiconductor layer, andwherein the third semiconductor layer has an impurity concentrationhigher than an impurity concentration of the second semiconductor layer;an insulator disposed over at least a portion of a light incidentsurface of the avalanche photodiode; and a resistor disposed over aleast a portion of the insulator and that extends along a space betweenadjacent avalanche photodiodes of the plurality of photodiodes, whereinthe resistor is connected to the avalanche photodiode, and wherein theresistor includes a linear portion.
 2. The avalanche photodiode arrayaccording to claim 1, wherein the resistor has a rectangular ring-likeshape that at least partially extends around the space between adjacentavalanche photodiodes of the plurality of photodiodes, and wherein thelinear portion is part of the rectangular ring-like shape.
 3. Theavalanche photodiode array according to claim 1, wherein a conductivitytype of the second semiconductor layer is the first conductivity type.4. The avalanche photodiode array according to claim 1, wherein aconductivity type of the second semiconductor layer is the secondconductivity type.
 5. The avalanche photodiode array according to claim1, wherein the first conductivity type is n-type and the secondconductivity type is p-type.
 6. The avalanche photodiode array accordingto claim 1, wherein the third semiconductor layer is disposed inside atleast a portion of the second semiconductor layer.